The single fibre EMG differences during motor axonal stimulation at different firing rates were studied in myasthenic patients:15 with, and 15 without thymoma. 10 healthy volunteers were also examined. Conventional repetitive stimulation EMG as well as SFEMG during weak voluntary contraction and on 10 and 20Hz stimulation was performed in every patient. The mean jitter in the control group was 30 ?s on voluntary contraction and about 22 ?s at 10 as well as at 20 Hz stimulation. In both groups of myasthenic patients under consideration SFEMG on voluntary contraction detected neuromuscular transmission disturbances of various degree. The results obtained at motor axonal stimulation (10 and 20 Hz) were unhomogeneous. In both groups jitter seemed to be slightly shorter at higher (20 HZ) frequency stimulation, probably due to facilitation but differences were significant.